The prior art has seen a wide variety of different approaches to consolidating proppants injected into fractures.
Hydraulic fracturing of a formation adjacent a borehole increases productivity of desirable hydrocarbonaceous fluids from the subterranean formation by creating channels of high fluid conductivity. In fracturing of formations, two major difficulties have been encountered. The first of these is proppant flowback. This decreases the amount of proppant holding the fractures pen, allowing closing of the fractures and causing reduced permeability. The second problem is sand production from the subterranean formation. The produced sand tends to lower productivity, also.
Resin-coated proppants which have the ability to consolidate have the potential to minimize both problems. Most of the commercial resin-coated proppants used today are heat cured and are therefore consolidatable only at high temperatures and pressures found in deep wells.
This leaves a long felt need for consolidation of proppants about low temperature wells such as shallow wells and those found in Alaska.
It is desirable to have relatively high strength so as to resist crushing of the proppant when consolidated. In this way, the proppant is retained in the formation, is not produced back into the borehole or the like. Resin-coated proppants consolidated downhole have been used to offset this problem.
Any resin-coated proppant should be compatible with the fracturing fluids used to carry it downhole and be as economical as possible. Moreover, extra steps should be curtailed when feasible and eliminated where possible since the additional steps that may be required adds to the expense.
The closest prior art that the inventors are aware of is the patent application on which one of the co-inventors is also co-inventor entitled "CONSOLIDATION OF HYDRAULIC FRACTURES EMPLOYING A POLYURETHANE RESIN", Ser. No. 410,859, now U.S. Pat. No. 5,049,608, filed Sep. 22, 1989. That application is assigned to the same assignee as is this application.
The prior art has ranged from U.S. Pat. No. 3,851,479, showing sealed porous earth formations where void spaces are reduced with the treatment with aqueous solutions containing hydroxyl, polyisocyanurates, and polyvinyl alcohol, through U.S. Pat. No. 4,114,382, concerned with consolidating geological formations with a polyol and a special 1,2-alkylene oxide, to recent patents; such as, U.S. Pat. No. 4,719,002, describing a method for preparing a molded reaction injection rigid non-cellular polyisocyanurates, and U.S. Pat. No. 4,731,427, describing a method of preparing molded reaction injection rigid polyurethane modified isocyanurate polymer composition. While these patents are pertinent chemically in that the reactants initially are similar, the result is vastly different from the invention herein.
It is desirable that any method of coating a proppant to provide downhole consolidation have the following features not heretofore provided:
1. A problem has been that, when simulating shearing action that the coated proppant will undergo before or during pumping downhole, the turbulent motion should not cause premature polymerization of the resin. Expressed otherwise, polymerization should not be occurring at this time. It is desirable that a reduced degree of reactivity of any coating be achieved so it does not come off the proppant when admixed with the carrier;
2. It is desirable that the system for consolidating proppant be applicable to either a one step method in which a catalyst is admixed with the admixture of diluent, and proppant or a two step method in which the sand that is coated with a capped reactant and diluent, .can be injected and then a catalyst overflushed, or flushed, as a minor proportion of a carrier liquid. It is also desirable that the coated proppant have properties set forth in a co-pending patent filed the same date as the earlier cited Ser. No. 410,859 by Robin Wiser-Halladay as inventor and entitled "A POLYURETHANE QUASI PREPOLYMER FOR PROPPANT CONSOLIDATION", U.S. Pat. No. 4,967,830 . The descriptive matter of that patent is incorporated herein in abbreviated form so that the reader will not have to advert to that patent; but, in any event, is incorporated herein by reference for any details that may have inadvertently been omitted herefrom;
3. The original reactants material should be relatively safe and not reactive with moisture, hence a blocked or "capped" type diisocyanate is preferred. The block, or cap can then be removed to react in situ, once the fluid containing the capped diisocyanate is in place.
The prior art has failed to provide the foregoing delineated features and to provide a proppant that can be cured in situ after it is emplaced in the formation, even in low temperature wells.